Electrolytes used in electroplating copper into damascene interconnects typically contain a copper salt, an acid, halide ions, an accelerator, a suppressor and a leveler. The copper salt is the copper source for the deposition. Acid is generally used to control the conductivity of the plating bath. Halide ions may act as bridges to assist the adsorption of certain organic additives (e.g., accelerator, suppressor and/or leveler) onto a substrate surface to encourage a bottom-up fill mechanism, described herein.
Example copper salts include, but are not limited to, copper sulfate, copper methanesulfonate, copper pyrophosphate, copper propanesulfonate, etc. As used herein, the concentration of copper ions reflects the concentration (mass per volume) of copper cations, and does not include the mass of any anions associated with the copper cations. Example acids include, but are not limited to, sulfuric acid and methanesulfonic acid. As used herein, the concentration of acid reflects the concentration (mass per volume) of the entire acid molecule, not the mass of hydrogen cations alone. Example halide ions include, but are not limited to, chloride, bromide, iodide, and combinations thereof.
It may be desirable to strongly polarize the substrate in the cathodic direction, particularly during the initial stages of electroplating onto a seed layer. Such polarization may protect the seed layer from dissolution. One way to achieve such polarization is by providing a strong “suppressor” in the electrolyte.
FIG. 2 shows a copper wafer 201 plated in an electrolyte containing a strong suppressor. It is a high molecular weight suppressor with high ratio of hydrophobic propylene oxide versus hydrophilic ethylene oxide The cloud point of the electrolyte is 27° C., and plating occurred at 21° C. The plated wafer contains visible streaks caused by non-uniform suppressor adsorption on the wafer due to suppressor agglomeration. The use of some electrolytes may result in more subtle forms of such defects that are not detectable by visual inspection, but which are detectable through common defect metrology such as the AIT, SP1, or SP2 series of tools from KLA-Tencor of San Jose, Calif.
Another technique for increasing the polarization of a substrate is to increase halide ion concentrations or change the halide ion composition. This technique is further described in U.S. Pat. No. 8,268,155, incorporated by reference herein. The halide may affect the suppressing effect of a suppressor or other additive. However, the increase in overpotential that may be gained by changing halide ion concentration or composition is limited, and may not sufficient to provide the conditions needed for a uniform, reproducible fill of small 10-20 nm features. Further, the concentration of halide ions should be relatively low in the electrolyte in order to avoid incorporation of the halides into the plated films or the formation of center voids due to insufficient bottom-up fill by over-suppression of copper deposition.
While conventional electroplating solutions have worked well for previous generations of damascene interconnect fabrication processes, new challenges are emerging as smaller features and thinner seed layers are used. Thus, it has recently been found that there exists a need for a method of electroplating damascene interconnects that better protects the seed layer from dissolution and allows the deposition to occur over a longer timeframe to achieve a more reproducible, more uniform fill of features.